In most microscopes, the visible light spectrum is used for imaging. X-ray microscopes are known. Two principal advantages of an x-ray microscope over a visible light microscope are (1) better potential resolution of extremely small features due to shorter wavelengths; and (2) some internal features can be observed which cannot be seen with a visible light microscope.
Most x-ray imaging devices involve directing a beam of x-rays through an object onto a phosphor screen which converts each x-ray photon into a large number of visible photons. The visible photons expose a sheet of photographic film placed close to the phosphor thus forming an image of the attenuation of x-rays passing through the object.
There are several limitations to film-screen x-ray devices. A major limitation is that the film serves the combined purpose of both the image acquisition function and the image display function. In addition, the range of contrast or latitude of the film is too limited to display the entire range of contrast in many objects of interest. Because of the limited latitude and dual acquisition/display function of film, a film-screen x-ray is often overexposed in one area and underexposed in another area due to the thickness and composition variations of the object across the image. The gray-scale level of x-ray film has a sigmoidal response as a function of exposure which results in difficulties in distinguishing contrast differences at the extremes of the exposure range; that is, in the most radiodense and in the most radiolucent areas of the image.
Digital x-ray techniques have been proposed as a technology which replaces the phosphor/film detector with a digital image detector, with the prospect of overcoming some of the limitations of film-screens in order to provide higher quality images. A potential advantage of digital x-ray technology involves the separation of the image acquisition function from the image display function. Digital detectors also provide a much greater range of contrast than film and the contrast response function is linear over the entire range. This would allow a digital detector to more easily distinguish subtle differences in attenuation of x-rays as they pass through various paths of the object. Differences in attenuation due to thickness and composition variations across the object can be subtracted out of the digital data in the computer and the residual contrast can then be optimized for the particular viewing mechanism, be it film or computer monitor. The residual contrast differences can then be analyzed to search for things of interest. Other advantages of digital x-ray technology include digital image archival and image transmission to remote location for viewing purposes.
Current digital x-ray devices have fairly limited resolution and so they are limited in their applications. What is needed is high resolution imaging devices capable of detecting microscopic internal features.